1. Field of the Invention
The invention concerns a hand-held barcode label reader with an oblong read window which is offered up substantially parallel to the label with a longer dimension transverse to the bars. The reader includes a lens system with an optical axis normal to the window at its center and adapted to project a conjugate image of the label onto a photosensor strip. The photosensor strip comprises a multiplicity of receiver elements which are electrically scanned sequentially and aligned in a sighting plane containing the optical axis and parallel to the longer dimension of the window and a light source outside the sighting plane emitting through the window towards the label a beam which is spread out parallel to the longer dimension of the window.
2. Description of the Prior Art
The barcode system is extremely widely used to show the reference codes of articles of all kinds in a form suitable for automatic capture so that these reference codes can be identified without a person having to read them visually. The product details are coded on a rectangular label by a succession of straight elements or bars transverse to the direction of the sequence and alternately dark and light, having widths which are integer multiples of a unit width (the multiplying factor is usually between 1 and 3). The label is read by optically scanning it in the longitudinal direction at approximately constant speed to produce an electrical signal embodying the reference code carried by the label and suitable for computer processing.
Probes, which are moved by hand to scan the label, have been used for reading the labels; the difficulty of performing the manual scanning at constant speed makes these probes difficult to use efficiently; the scanning often has to be repeated several times and the response is slow. The trend is to discard this type of probe in favor of readers with a built-in scanner.
There are two main types of readers with a built-in scanner, "hand-held" readers so-called, as they are small in size, are moved over the label, and fixed readers have the article moved over a read window.
In fixed readers, a concentrated narrow light beam is deflected by rotating optics, and a photosensitive receiver registers a light signal formed by the light backscattered from the concentrated beam scanning the label.
The general construction of modern hand-held readers is explained above. The label is illuminated by a luminous flux that is practically fixed during reading, and its image is projected by the lens system onto the photosensor strip. The receiver elements are scanned sequentially to form the electrical signal representing the code carried by the label. The scanning is repeated cyclically, and as the reader moves towards the label, the image formed by the lens system on the photosensor strip becomes progressively more clearly focussed until the label is in the conjugate plane of the photosensor strip. As a corollary to this, the scanning signal becomes progressively better defined. The photosensor strip control device analyzes the signal and validates it, as soon as its definition is satisfactory.
The specifications of the lens system (focal length and aperture) define a depth of field within which the label code can be read unambiguously; this depth of field can be determined by the man skilled in the art.
However, modern hand-held readers have a depth of field which is limited for a different reason, namely the conditions under which the label is illuminated. The reader must be relatively insensitive to the ambient light, which implies the use of a light source integrated into the reader with a spectrum different from that of the ambient lighting, and this light source must direct onto the label a luminous flux sufficiently intense to enable the use of a small aperture lens system to provide a satisfactory depth of field.
The light source must be outside the sighting plane, as it would otherwise intercept the image of the label intended to fall onto the photosensor strip if the light source were in front of the strip, while the latter would intercept the beam emitted by the source if the latter were behind the strip.
The median plane, along which the beam from the light source is spread, conventionally forms with the sighting plane a dihedron, whose line of intersection lies parallel to the longer dimension of the read window in the conjugate plane of the photosensor strip. If the label moves away from this conjugate plane, the section of the label in the sighting plane, which is imaged on the strip, is illuminated only by a marginal portion of the beam emitted by the source; the useful illumination reduces with the distance between the label and the conjugate plane of the strip at a rate which is directly proportional to the angle of the dihedron between the median plane of the beam and the sighting plane. The light backscattered by the label has a maximum in a plane symmetrical to the incident plane of the beam relative to a plane normal to the label. All these factors contribute to reducing the usable depth of field of conventional hand-held readers.
If the beam from the light source is concentrated to increase the illumination near the median plane of the beam, the decrease in the illumination on moving away from the median plane is accentuated, and the gain in terms of depth of field is minimal; there may even be no improvement at all or a deterioration.